Sorry but I have to disagree here. The amount of energy required to attack these intruders is going to be capped at some point regardless of how long you have played. 

There is a max to the Endurance-Related Hero Skills

There is a max to the Endurance-Related Invader Knowledge

By time KVK/CVC event starts, all kingdoms should have had a cycle of Royal Guardsman Invaders (takes a day or so to max legendary the set).

The energy requirements for fighting and killing an Uber Invader far exceed that of a Level 6 (though if you do it right, it can be made pretty damn efficient).

So yeah ... a 30 day old KD has the same opportunity to kill these invaders with the same efficiency as a KD that is years old. Probably more so, as I expect they are all fresher with those cheap $5 - $10 packs full of Hero Energy Potions....